Traditionally, the recording and reading of data in tracks on magnetic storage media can require precise positioning of magnetic read/write heads. To facilitate the precise positioning, magnetic storage media can include a servo track read by a servo control system. U.S. Pat. No. 5,689,384 includes details of background servo timing methods and devices, and is incorporated herein by reference.
Track following servo systems have kept the industry on track to achieve one terabyte (“TB”) of storage on a commercial single reel tape cartridge. It appears, however, that modifications may be required to move beyond 1 TB. Longitudinal errors in placement of the servo pattern can generate false position information during drive servo operation. The magnitude of these errors can become unacceptable at the high track densities of multi-terabyte data cartridges.
Time-based servo bands, which are written to tape during the manufacturing process, can be used as a reference to position all data tracks for the life of the cartridge. Time-based servo bands can include a series of repeating patterns (e.g., servo frames) down the length of a tape. In a simplified version of this pattern, each frame can be made up of two magnetic stripes written at an angle with respect to one another (e.g., a “/\” shape). FIG. 1 illustrates two frames 110 in the “/\” shape. The servo pattern can be decoded in a drive by measuring the distance (a) 120 between frame stripes at the servo read gap position while tape is transported longitudinally across the recording head (e.g., in direction 150). A geometric transformation can then yield the vertical position (y) 130 of the read gap 140 with respect to each servo frame on tape.
Servo-writing for magnetic tape can utilize a ring head technology to write longitudinal timing-based servo patterns on tape. This has traditionally been useful for writing metal particle (“MP”) media, with a magnetic moment substantially in plane with the media. However, there also exist perpendicular media with the magnetic moment perpendicular to the plane of the tape, or other media surface. Barium-Ferrite (“BaFe”) is an example of such a tape medium. BaFe media are described in U.S. Pat. Nos. 7,132,164, 7,381,482, 4,690,768, 4,493,779, and 4,493,874, which are incorporated herein by reference. Perpendicular media can be preferential in some applications, and may provide a greater data density.
Traditional ring head designs can write timing-based servo patterns to magnetic tape. FIG. 2A illustrates one such traditional ring head design, 205. When pulsed with write current (e.g., through coil 210), these traditional heads 205 can generate a magnetic field 200 that is mainly longitudinal between the two poles and vertical (and opposite) very near the poles. When writing media with a planer magnetic moment (e.g., a magnetic moment in the plane of the media), the resulting magnetization of the media can create a useful read-back signal with sharp transitions at the boundaries of the regions magnetically written by the write head, e.g., as illustrated in FIG. 2B. However, such a traditional ring head design does not produce the same results for perpendicular media, such as media 220 illustrated in FIG. 2A.
The magnetization state of oriented media 220 is illustrated in cross-section, and can be driven from left to right (or vice-versa) over ring head 205. Magnetization elements are shown in both a positive and negative orientation in AC erased area 221. Using a ring head 205 on AC erase media creates both a positive 223 and negative 227 magnetization region.
FIG. 2C illustrates a ring head 205 writing to less oriented media 222. The magnetization state of less oriented media 222 is AC erased (e.g., as shown in areas 221) with ring head 205 writing to area 224. As illustrated, the longitudinal nature of ring head field 210 can cause less oriented media 222 (e.g., media that is not completely perpendicular) to have a transition zone 224 between the positive and negative regions that shows a transition with a longitudinal component (e.g., a “leaning” orientation at the transition). The magnetization state of less oriented media 222, with transition area 224, is illustrated in perspective view in FIG. 2D, and again in cross-sectional view in FIG. 2E.
FIG. 2F illustrates a cross-sectional view of the magnetization state of DC erased media 225, with a ring head field 200 applied. As illustrated in DC erased areas 231, the erased media orientation is in a single direction, consistent with a DC current flowing through a coil in one direction. This can be compared to AC erased media, where erased areas (e.g., 221) are oriented in both directions, in approximately equal amounts, consistent with an AC current alternating through coil 210. Here, as compared to FIGS. 2A and 2C, the back half of the ring head field can have the same concentration (e.g., complete) as the erased areas 231. However, as shown in FIG. 2F, there can still be “fuzzy” or “leaning” transition lines in the written area.
Thus, there exists a need for new pole head designs for writing the timing based servo pattern on substantially perpendicular media.